The reassuring finding comes from analysis of long-term data on 21,457 women with breast cancer enrolled in clinical trials of tamoxifen.

"Substantially reduced mortality rates for breast cancer continue well beyond year 10, as a delayed effect of the greatly reduced [breast cancer] recurrence rates during years 0 to 9 [after about five years of tamoxifen therapy]," report Christina Davies, MD, and colleagues in the international Early Breast Cancer Trialists' Collaborative Group (EBCTCG).

One reason breast cancer is so deadly is that it often comes back after what a woman had hoped would be curative treatment. Tamoxifen reduces this risk.

This means many women's breast cancers are cured, suggest Stephen K. Chia, MD, of the University of British Columbia and Antonio C. Wolff, MD, of Johns Hopkins University, in an editorial accompanying the EBCTCG report in the July 29 online issue of The Lancet.

"Five years of tamoxifen can prevent a high proportion of recurrences and potentially cure many patients, rather than simply delay an inevitable event," Chia and Wolff note.

Tamoxifen works only on the most common form of breast cancer: the form that needs the female hormone estrogen in order to grow. These cancers carry proteins, called receptors, that bind estrogen. Tamoxifen makes it impossible for these receptors to bind estrogen.

Importantly, the EBCTCG study shows that tamoxifen works even on breast cancers that carry only small numbers of estrogen receptors.

"Therefore, another crucial message from this [study] is the need for accurate and sensitive [tests] ... to detect even low concentrations of estrogen receptor and thus further identify potential candidates for ... tamoxifen treatment," Chia and Wolff suggest.

Newer drugs, called aromatase inhibitors, also block estrogen. But these drugs don't work in premenopausal women. Moreover, it's still not known how long their benefits last.

"Access to accurate estrogen-receptor testing, and to tamoxifen (a relatively inexpensive drug) or other endocrine therapies is a public health imperative for all women with breast cancer," Chia and Wolff conclude.